To meet the continued fast growing demand of mobile data, the wireless industry needs solutions that can provide very high data rates in a coverage area to multiple users simultaneously including at cell edges at reasonable cost. Currently, the wireless telecom industry is focused on dense deployment of small cells, the so called ultra-dense networks, to increase spatial re-use of wireless spectrum as the solution for meeting the growing mobile data demand. Dense deployment of small cells requires a large number of backhauls and creates highly complex inter-cell interference. One solution to the interference problem is to require careful Radio Frequency (RF) measurement and planning and inter-cell coordination, which significantly increases the cost of deployment and reduces the spectral efficiency. Another solution is the Self-Organizing Network (SON) technology, which senses the RF environments, configures the small cells accordingly through interference and Tx management, coordinated transmission and handover. SON reduces the need for careful RF measurement and planning at the cost of increased management overhead and reduced spectral efficiency. The backhaul network to support a large number of small cells is expensive to be laid out.
Another method for increasing spatial re-use of wireless spectrum is MIMO, especially Multi-User MIMO (MU-MIMO). In a wireless communication system, a wireless node with multiple antennas, a Base Station (BS) or a User Equipment (UE), can use beamforming in downlink (DL) or uplink (UL) to increase the Signal-to-Noise Ratio (SNR) or Signal-to-Interference-plus-Noise Ratio (SINR), hence the data rate, of the links with other wireless nodes. MU-MIMO can beamform to multiple UEs simultaneously in a frequency and time block, e.g., a Resource Block (RB), i.e., using spatial multiplexing to provide capacity growth without the need of increasing the bandwidth. In a large-scale MIMO or massive MIMO system, a BS may be equipped with many tens to hundreds of antennas. In order for the BS to beamform to multiple UEs using the plural of antennas, the BS needs to know the DL channels to the UEs sufficiently accurately, e.g., the DL Channel State Information (CSI) of each UE. However, it is not efficient to obtain the DL CSI directly by sending reference pilots in the downlink because of two reasons: (1). The large number of antennas on the BS would cause large system overhead for reference signals in the downlink; (2). Dozens of bits are needed to quantize the CSI accurately, which causes overload of the feedback channel in the UL. Fortunately, the reciprocal property of an over the air wireless channel, such as in a Time-Division Duplexing (TDD) system or in a Frequency-Division Duplexing (FDD) system using switching to create channel reciprocity as described in our PCT application PCT/US14/71752 filed on Dec. 20, 2014, claiming the benefit of provisional patent application 61/919,032 filed on Dec. 20, 2013, can be employed to reduce the channel estimation overhead. In such a system, a UE sends a pilot signal, e.g., Sounding Reference Signal (SRS), which is received by all the antennas on the BS in the UL. The BS estimates the UL CSI through the received pilot signal and uses it to estimate the DL CSI based on channel reciprocity.